Cylindrical structures such as marine risers, umbilicals, cables, and pipelines will generally be subject to vortex-induced vibration excitation when immersed in a flowing fluid medium, or when moving relative to the fluid medium. This dynamic excitation can result in an amplified drag force and a decreased operating life due to fatigue.
In the offshore industry, cylindrical structures are used in production and drilling risers, pipelines near the seabed, seawater intakes, discharge lines, and many other similar structures. Generally, these cylindrical structures will be subject to VIV, which can be more problematic with greater flow/current speed and longer cylinder lengths.
Flow modification devices have been developed to reduce the level or severity of VIV on cylindrical structures. These flow modification devices aim to prevent coherent vortices from shedding about the cylindrical structure. U.S. Pat. No. 8,443,896 describes a flow modification device, in the form of a plurality of helical strakes, connectable to a marine riser. Helical strakes may reduce the severity of VIV to very small levels, but are generally large and not practical to handle. For example, drilling risers involve deployment, retrieval, and stacking operations which are not easily achieved with large helical strake devices.
U.S. Pat. Nos. 7,513,209 and 6,223,672 describe rigid fairing devices connectable to marine risers. Rigid fairings aim to suppress VIV by streamlining and delaying separation of the flow about the cylindrical structure. Rigid fairings are typically unidirectional devices and generally involve clamping the fairing to the cylindrical structure. The clamping process can be time consuming, which is disadvantageous in light of expensive production and operating schedules, for example, when undertaking drilling operations.
Vortex shedding about cylindrical structures is generally described in Flow-Induced Vibrations: An Engineering Guide (2005) by Naudacher, E. and Rockwell, D. from Dover Publications, which is incorporated herein by reference. Further flow modification devices and/or methods are disclosed in the following patent publications: FR 2367148, WO 2002/095278, and WO 2009/035481.
It is an object of the present invention to provide flow modification devices, systems, and methods which overcome, or at least ameliorate, one or more deficiencies of the prior art, or at least provides a useful alternative.
Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.